Process for preparation of rubber composition

ABSTRACT

A process for the preparation of a carbon black-containing rubber composition having improved green strength the rubber components of which consist of 80 - 40 percent by weight of butadiene-acrylonitrile copolymer rubber (A) and 20 - 60 percent by weight of isoprene-acrylonitrile copolymer rubber (B) or a carbon black-containing rubber composition the rubber components of which consist of 60 - 30 percent by weight of butadienestyrene copolymer rubber (A) and 40 - 70 percent by weight of natural rubber (B), wherein 100 parts by weight of rubber (A) are mixed with 30 - 100 parts by weight of carbon black, the mixture is milled and aged, and then it is mixed with rubber (B), and the resultant mixture is milled.

United States Patent [191 Todani et al.

[111 3,872,036 i451 Mar. 18, 1975 PROCESS FOR PREPARATION OF RUBBER COMPOSITION [75] Inventors: Yoshihiro Todani, Kamakura;

Nagatoshi Sugi; Kinro Hashimoto, both of Yokohama, all of Japan [73] Assignee: The Japanese Geon Co. Ltd., Tokyo,

Japan [22] Filed: Mar. 10, 1971 [21] Appl. N0.: 123,020

[30] Foreign Application Priority Data Mar. ll, 1970 Japan 45-20071 [52] US. Cl. 260/5, 260/41.5 R, 260/4l.5 MP,

260/894 [51] Int. Cl C08c 9/04, C080 ll/18,C08d 9/00 [58] Field of Search... 260/4l.5 MP, 894,5, 41.5 R

[56] References Cited UNITED STATES PATENTS 2,764,572 9/1956 Pechukas 260/ 8/l969 Forrester et al. 260/4L5 MP '10/1970 Morris ct al. 260/4l.5 MP

[57] ABSTRACT A process for the preparation of a carbon blackcontaining rubber composition having improved green strength the rubber components of which consist of 80 40 percent by weight of butadiene-acrylonitrile c0- polymer rubber (A) and 60 percent by weight of isoprene-acrylonitrile copolymer rubber (B) or a carbon black-containing rubber composition the rubber components of which consist of 60 percent by weight of butadiene-styrene copolymer rubber (A) and percent by weight of natural rubber (B), wherein parts by weight of rubber (A) are mixed with 30 100 parts by weight of carbon black, the mixture is milled and aged, and then it is mixed with rubber (B), and the resultant mixture is milled.

3 Claims, N0 Drawings PROCESS FOR PREPARATION OF RUBBER COMPOSITION The present invention relates to a process for the preparation of an improved rubber composition, and more particularly to a process for improving the green strength of a rubber composition comprising butadiene-acrylonitrile copolymer rubber, isopreneacrylonitrile copolymer rubber and carbon black or a rubber composition comprising butadiene-styrene cpolymer rubber, natural rubber and carbon black.

Heretofore, butadiene-acrylonitrile copolymer rubber (which will herinafter be referred to as NBR) and isoprene-acrylonitrile copolymer rubber (which will hereinafter be referred to as NIR) have been widely used as special rubbers, and butadiene-styrene copolymer rubber (which will hereinafter be referred to as SBR) and natural rubber have been widely used as rubbers for universal use. These rubbers have their respective characteristics, and they are not only used alone, but also used as blends having properties superior to the individual component rubbers. When a rubber is processed, a reinforcing agent such as carbon black is generally incorporated, and further it is cross-linked with a cross-linking agent, but where not less than two kinds of rubber are blended, the incorporation of carbon black has usually been carried out after the two kinds of rubbers are blended togther or simulaneously with the blending ehblending of the rubbers. It has also been known previously that butadiene type synthetic rubbers such as NBR and SBR have inferior green strength as compared with natural rubber and NIR, and for this reason there have been many problems, such as breakage of rubber and ustabilized shape, in the manufacture of rubber products especially in the step of tire fabrication This defect of butadiene type synthetic rubbers cannot be removed by blending these with natural rubber or NIR and carbon black by the conventional procedure. Mainly for this reason, the proportion of butadiene type synthetic rubbers used has not been able to be raised in the field of blended rubbers. Accordingly, various studies have been made on the improvement of green strength of butadiene type synthetic rubbers.

Generally, it is considered that the green strength of a rubber depends on the molecular weight distribution of the rubber, and it can be improved by the presence of a high molecular weight polymer in the rubber. As a matter of fact, if an ultrahigh molecular weight polymer is blended with a rubber, the green strength of the rubber can be remarkably improved. However, if carbon black is added to this blend, this effect is entirely lost. In other words, a method for the improvement of green strength in a rubber containing no carbon black does not apply to the improvement of green strength in a rubber blend containing carbon black. When a rubber is blended with carbon black, polymers are adsorbed to the surface of carbon black, and at this time polymers of higher molecular weight are selectively absorbed. This phenonmenon has long been known. The reason why the effect of green strength improvement is lost when carbon black is incorporated into a rubber whose green strength has been improved by an ultrahigh molecular weight polymer is probably that the untrahigh molecular weight polymer is selectively absorbed onto the surface of carbon black to form a carbon gel.

Accordingly, one object of the present invention is to provide a process for improving the green strength of a rubber composition comprising NBR, NIR and carbon black or a rubber composition comprising SBR, natural rubber and carbon black.

The other objects of the present invention will be made clear by the following description.

It has been discovered that these objects of the present invention can be attained by compounding I00 parts by weight of rubber (A) with 30 100 parts by weight of carbon black, milling and ageing the mixture, and then compounding it wih rubber (B) and milling the mixture at the time of producing a carbon blackcontaining rubber composition the rubber components of which consist of 80 40 percent by weight of NBR (A) and 20 60 percent by weight of NIR (B) or a carbon black-containing rubber composition the rubber components of which consist of 60 30 percent by weight of SBR (A) and 40 percent by weight of natural rubber (B).

As described above, rubber (A) (NBR or SBR) is known to have inferior green strength, and rubber (B) (NIR or natural rubber) is known to have superior green strength. Accordingly, rubber compositions obtained by blending only rubber (A) and rubber (B) together can possess improved green strength. However, rubber compositions comprising rubber (A), rubber (B) and carbon black do not show improved green strength for the above-described reason, as long as the carbon black is blended by the conventional procedures.

However, the green strength of rubber compositions can be remarkably improved by the method of the pres ent invention, namely by first compounding rubber (A) (NBR or SBR) which has inferior green strength with carbon black, milling and ageing the mixture, thereby allowing the surface of carbon black to adsorb polymers to form a carbon gel (which blend will herinafter be referred to as gelled carbon black master batch or GCM), and then compounding the blend with rubber (B) (NIR or natural rubber) having superior green strength.

The method of the present invention can be applied to rubber compositions the rubber components of which consist of 40 percent by weight of NBR and 20 60 percent by weight of NIR and rubber compositions the rubber components of which consist of 60 30 percent by weight of SBR and 40 70 percent by weight of natural rubber. Namely, if NBR exceeds 80 percent by weight or SBR exceeds 60 percent by weight, the effect of green strength improvement by addition of NIR or natural rubber can not be expected, whereas if NBR is lower than 40 percent by weight or SBR is lower than 30 percent by weight, the resultant blend does not substantially differ from the case where NIR or natural rubber is used alone, and the effect of the present invention can not be achieved. Further, the amount of carbon black to be used for the preparation of the gelled carbon black master batch is 30 parts by weight per 100 parts by weight of NBR or SBR. When the amount of carbon black is less than 30 parts by weight, green strength can not be improved, whereas if it exceeds 100 parts by weight, an over carbon gelling (like carbn scorching) phenomenon occurs, dispersion becomes poor in the final composition, and this adversely affects the physical properties and processability of the rubber. Further, in order to prepare that NBR or SBR should be compounded and milled with carbon black and the blend should be aged sufficiently, as described above. Ageing here means allow- Next, the present invention will be illustrated by nonlimitative Examples.

EXAMPLE 1 ing the blend to stand at room temperature usually for 5 notless than 12 hours, preferably not less than 1 day.

If NIR or natural rubber is incorporated into the blend clomposmons A and B each conslstmg of 75 parts by without ageing the blend, green strength can not be im- Weght of NBR (Hycar [0521 f by the proved Geon Co., Ltd.), 25 parts by weight of NIR (Polysar NBR, NR and SBR used in the method of the 833 made by Polymer Corporation Ltd.) and 30 parts present invention may be produced by the convenby welght f HAF carbon black were prepared m tional emulsion polymerization technique. The acrylocordanc? W1th the followmg procedures nitrile contents of NBR and NIR are usually 10 60 Composltlon A:

percent, f bl 20 40 percent, and the Styrene NBR andcarbon black were preliminarily milled to- Comem f SBR is usually 10 5 percent, preferably 15 gether by 6-inch rolls at about 60C. for 7 minutes, the 5 30 percent. s Synthetic rubbers and natural blend was allowed to stand at room temperature for l0 rubber may have suitable molecular weights, and those days and then NIR was mined therewith y rolls having a Mooney viscosity of 200 are generally about used. Composition B:

Preparation of the gelled carbn black master batch is 20 NBR and NIR were milled ogether by 6-lnCh rolls at usually carried out by a mechanical mixer such as roll about for 2 s, and then HAF Carbon black mill r B b mix r, b t i may l b i d out was added to it and the mixture was milled for 8 minby incorporating an aqueous or non-aqueous dispersion Utesf b n bl ck i t a lat x f NBR or SBR, th b These compositions were shaped by 6-inch rolls into obtaining a socalled wet process carbon black mater sheets having a thickness of about 2.5 mm. Each sheet batch. However, in case this wet process is followed, was Placed in a metal mould having a thlckngss of 2 the formation of carbon gel is less since the rubber has mm, and n as eated at 100 110C. for 60 minnot been subjected to a milling operation after drying, Utes er a pressure of 75 tons and then the same and so no noticeable effect can be achieved. For this p su e as applied thereto for 180 minutes. while reason, i i necessary h th bb after d i cooling it gradually, to form a sheet having a thickness should be milled with an extruder, etc., or mechanical of about 2 mm and a moo Surface. Out of this sheet illi b means f a ll ill or a B b mixer dumbbell test pieces as specified in S No. 3 were should be effected prior to blending with the other rubpunched in parallel With the rolling Ol'lfil'llZlIlOfl. After b d h i h ld b d ffi i l this test piece was allowed to stand for one day. it was The rubber composition of the invention can be prostretched by an lnstron tension tester at a stretch velocduced by compounding the gelled carbon black master of 500 mm/mln" and the relation between elongabatch thus obtained with NIR or natural rubber and if tion and tension W218 measured. To measure the elongad i d, carbon bl k, d h i ll illi h tion accurately, measure lines were drawn on the test mixture. At this time, a part of NIR or natural rubber 40 P ece a ntervals Of 20 mm, and the change ofdistance may be replaced by NBR or SBR, respectively. between the measure lines was read, and the corre- F th b id carbon bl k an di ft i sponding tension was measured. Tensile (stress per unit agent, process oil, etc. may also be incorporated accross-sectional area of the unstretched test piece) at cording to need in the rubber composition of the preseach ratio of elongation is shown in the following Table ent invention. Usually 40 150 parts by weight of car- 5 1. From this Table it is found that composition A prebon black and 30 100 parts by weight of process oil pared in accordance with the method of the present inper 100 parts by weight of total rubber are used. vention shows a higher tension at every ratio of elonga- The carbon black in the rubber composition of the tion, especially at elongation ratios of 10 20, as compresent invention has preliminarily been saturated with pared wth composition B prepared by the conventional NBR or SBR having inferior green strength unlike the method.

Table 1 Ratio of elongation Tension (times) 1 2 4 6 s 10 (kg/m2) 12 14 I6 1 s 20 (MDi+4/l00C.)

Composition A (71.0) 9.5 8.5 7.4 7.2 7.2 7.3 7.2 6.8 6.5 6.0 5.6 Composition B case of the conventional blending process, and so NIR EXAMPLE 2 or natural rubber which orlginally has superior green strength can act upon it effectively. Therefore, accord- An eXperimental Banbury mixer having a capacity of ing to the present invention, a rubber composition which as a whole has very superior green strength characteristic can be obtained.

0.8 liter was used, and 4 kinds of gelled carbon black master batches (GCM) l IV were prepared under the conditions of Table 2 and those following.

6 Table 2 Table 3 Composition of Gelled Carbon Black Master Batches R I ip for mpound M I CM I G eci e GC G 1 CM m GCM W p Natural rubbcr* GCM I- IV HAF Carbon Oil-extended SBR 137.5 137.5 137.5 137.5 (Wlrts by P" (Wms v poun (Nlpol J) 332 by weight) by weight) HAF carbon black 40 70 95 125 111 100 111 170 20 Naphthenic oil/SBR 37..5/l00. made by Japanese Geon Co.. Ltd. Iv [00 IV 190 Mooney viscosity (ML /1011c i= 43 o extended W V 100 SBR I00 90 Mooney Viscosity 60.

The initial side wall temperature and the number of rotation of the rotor were made 60C. and 180 rpm, re- Dumbbell test p1eces were produced out of these spectively, and the heating jacket was controlled so as compounds under the same conditions as those of Exto maintain the temperature of the stock at 90- 100C. ample 1. nd hen the relation between elongation and One minute after the oil-extended SBR was placed in tension was measured. The results were as shown in the the Banbury mixer, carbon black was added, and the following Table 4. milling was continued for 5 minutes. Further, each ofcompounds II and V were suspended After the gelled carbon black master batches I IV in a times amount of benzone by stirring the mixture thus prepared were allowed to stand at room temperafor 24 hours, and then this suspension was subjected to ture for 7 days, they were milled with natural rubber the action ofa 1200 rpm centrifuge to precipitate car- (RSS No. l) and carbon black by using a 0.8 liter Banbon gel. The carbon gel was fully dried, decomposed by bury mixer, and finally compounds I IV each consist- 25 heating, and then the gas evolved was analyzed by gas ing of 50 parts by weight of oil-extended SBR, 50 parts chromatography to determine its composition. The reby weight of natural rubber and parts by weight of sults were as shown in the following Table 5.

Table 4 Elongation (times) Elongation Tension 0.5 l 2 3 4 5 6 7 8 9 10 at breakage (kg/Cm (times) Compound I (51.0) 3.6 3.6 3.3 3.3 3.6 3.7 4.0 4.2 4.5 4.7 4.9 10.1 Compound II (49.5) 3.8 3.6 3.4 3.5 3.8 4.2 4.5 5.0 5.4 5.6 5.8 10.4 Compound III (46.9) 3.6 3.5 3.3 3.3 3.5 3.6 3.9 4.1 4.3 4.4 4.4 10.0 Compound IV 3.8 3.7 3.6 3.6 3.7 3.8 3.7 6.0

(43.0) Compound V HAF carbon black were obtained. Further, in this case, Table 5 the Banbury milling conditions were the same as those for the preparation of said GCM. Namely, first the Composition fc b Gel gelled carbon black master batch was milled with mas- 5BR atu u ber t1cated natural rubber for 1 minute, and then carbon Compound H 52 48 (7c black was added thereto, and milling was carried out Compound V 30 70 7r for 5 minutes.

thB other hand! for compaflsonls k Compound From Table 4, it is seen that a superior green strength V prepared by the Ordinary mlumg meth0d characteristic can be obtained by preliminarily producwherein the whole amount of carbon blacl is added ing a Carbon master batch from 100 parts by weight of after oil-extended SBR has been blended with natural SBR d 40 95 parts by weight of carbon black. From rubber- Table 5, it is found that in the compound prepared by The reclPe for the final Compounds was as Show In the method of the present invention, the amount of natthe following Table 3.

ural rubber absorbed by carbon black is small.

EXAMPLE 3 Four kinds of compound Vl IX composed of oilextended SBR. natural rubber and carbon black according to the recipe shown in the following Table 6 were prepared by using a Banbury mixer in the same manner as in Example 2, and then the relation between elongation and tension was measured in the same manner as in Example 1. The results were as shown in the following Table 7.

Table 6 Recipe for Compound Natural rubber (parts by weight) (parts by weight) Carbon black (parts by weight) Com- GCM [1* pound V1 90 40 Com- Oil-extended SBR** pound V11 60 40 Com- GCM 11* pound VH1 60 60 Com- Oil-extended SBR** pound The same with GCM ll of Example 2 (which was prepared by mixing 137.5 parts by weight of oil'extended SBR with 70 parts by weight of carbon black. milling the mixture, and then allowing it to stand for 7 days).

** Nipol 1778 J.

Table 7 Elongation ratio (times) Elongation Tension 0.5 l 2 3 4 6 8 l0 12 at breakage (kg/cm (times) (ML /IOOT.

Compound Vl (53.5) 3.6 3.5 3.0 3.0 3.1 3.3 3.7 3.9 4.0 12.5 Compound Vll (53.5) 3.6 3.4 3.0 2.8 2.6 2.4 2.3 2.] 11.0 Compound Vlll (53.0) 4.2 4.6 5.2 6.4 8.2 11.3 14.0 9.0 Compound lX From the above results, it is found that a very superior green strength characteristic is shown when a compound of 60 percent by weight of oil-extended SBR (about 43 percent calculatex as SBR) and percent by weight of natural rubber or a compound of 40 percent by weight of oil-extended SBR (about 29 percent calculated as SBR) and 60 percent by weight of natural rubber is prepared in accordance with the method of the present invention.

EXAMPLE 4 compounds each containing equal amounts of oilextended SBR, natural rubber and carbon black were prepared by using a 1.8 liter Banbury mixer.

c a rbon black thcreto. and then milling the mixture. Compound Xll was prepared in accordance with the conventional millng method, namely by milling oilextended SBR with natural rubber. and then milling the Table 8 Elongation ratio (times) Elongation Tension 0.5 l 2 3 4 5 6 7 8 9 at breakage (kglcm (times) (ML /l00C.)

Compound X 3.1 3.2 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 9.2

47.0) Compound XI (52.0) 3.5 3.3 2.9 2.8 2.8 2.9 3.0 3.1 3.2 3.3 9.0 Compound Xll EXAMPLE Oil-extended-SBR latex (Nipol 1712 made by Japanese Geon Co., Ltd; aromatic oil/SBR 37.5/100) whose solid content was 100 parts by weight was mixed and stirred with an aqueous suspension'of 50 parts by weight of HAF carbon black, the mixture was coagulated by an aqueous salt solution-aqueous sulphuric acid solution, and then it was dried with hot wind (the rubber carbon black mixture thus obtained will hereinafter be referred to as WCM). A part of WCM was milled with 6-inch rolls for about 2 minutes (this product will hereinafter be referred to as WCMR). 75 parts by weight of each of these WCM and WCMR were allowed to stand for 1 day, and then each was milled with 50 parts by weight of natural rubber and 25 parts by weight of carbon black by use of a Banbury mixer in the same manner as in Exmuple 2. and thus compounds XIII and XIV containing equal amounts of oil-extended SBR, natural rubber and carbon black were prepared The relation between elongation and tension in these compounds was measured in the same manner as in Example l. The results were as shown in the following Table 9.

From Table 9, it is found that compound XIV of the and aging the resulting mixture by allowing said mixture to stand at room temperature for a period of at least 12 hours, and then mixing the milled and aged mixture with said rubber (B), and milling the resultant mixture.

2.'In a process for the preparation of a carbon blackcontaining rubber composition wherein the rubber components of said composition consist of 80-40 percent by weight of butadiene-acrylonitrile copolymer rubber (A) and -60 percent by weight of isopreneacrylonitrile copolymer rubber (B) or 60-30 percent by weight of butadiene-styrene copolymer rubber (A) and 40-70 percent by weight of natural rubber (B), the improvement wherein a rubber composition having improved green strength is obtained by first compounding said rubber (A) with carbn black. milling the resulting mixture and aging the resulting mixture by allowing said mixture to stand at room temperature for a period of at least 12 hours, and then mixing the resulting gelled carbon black master batch with said rubber (B) and carbon black, and milling the resultant mixture, the total amount of carbon black used being 40-150 parts by weight per 100 parts by weight of the total rubber.

3. In a process for the preparation o f a carbon blackpresent invention which h u d the gell d carbon containing rubber composition wherein the rubber black master batch (WCMR) obtained by application Components of said composition consist of 80-40 perof shearing stress has a superior green strength characcent by weight of butadiene-acrylonitrile copolymer teristie as compared with compound XIII which has rubber (A) and 20-60 percent by weight of isopreneused th conventional wet process carbon black master acrylonitrile copolymer rubber (B) or 60-30 percent batch (WCM). by weight of butadiene-styrene copolymer rubber (A) Table 9 7 Elongation l'allO (times) Tension 0 5 l 2 3 4 5 7 9 l l Elongation (kg/cm) at breakage (times) (ML, /l00C.)

Compound XIII (48.0) 3.6 3.5 3.0 2.9 2.9 3.0 3.2 3.5 3.8 l 1.0 Compound XIV We claim: 4 and 40-70 percent by weight of natural rubber (B), the 1. In a process for the preparation of a carbon blackimprovement wherein a rubber composition having imcontaining rubber composition wherein the rubber proved green strength is obtained by first mixing a latex components of said composition consist of 80-40 perof said rubber (A) with a dispersion of carbon black, cent by weight of butadiene-acrylonitrile copolymer coagulating, drying and milling the resulting mixture rubber (A) and 20-60 percent by weight of isopreneand aging the resulting mixture by allowing said mixacrylonitrile copolymer rubber (B) or 60-30 percent ture to stand at room temperature for a period of at g by weight of butadiene-styrene copolyner (A) and least 12 hours, and then mixing the resulting gelled car- 40-70 percent by weight of natural rubber (B), the i bon black master batch with said rubber (B) and carprovement wherein a rubber composition having i bon black, and milling the resultant mixture, the total proved green strength is obtained by mixing 100 parts amount of Carbon black "S being 40450 Parts y by weight of said rubber (A) with 30-100 parts by Weight P 100 Parts by Welght of the total rubberweight of carbon black, milling the resulting mixture 

1. IN A PROCESS FOR THE PREPARATION OF A CARBON BLACKCONTAINING RUBBER COMPOSITION WHEREIN THE RUBBER COMPONENTS OF SAID COMPOSITION CONSIST OF 80-40 PERCENT BY WEIGHT OF BUTADIENE-ACRYLONITRILE COPOLYMER RUBBER (A) AND 20-60 PERCENT BY WEIGHT OF ISOPRENE-ACRYLONITRILE COPOLYMER RUBBER (B) OR 60-30 PERCENT BY WEIGHT OF BUTADIENE-STYRENE COPOLYNER (A) AND 40-70 PERCENT BY WEIGHT OF NATURAL RUBBER (B), THE IMPROVEMENT WHEREIN A RUBBER COMPOSITION HAVING IMPROVED GREEN STRENGTH IS OBTAINED BY MIXING 100 PARTS BY WEIGHT OF SAID RUBBER (A) WITH 30-100 PARTS BY WEIGHT OF CARBON BLACK, MILLING THE RESULTING MIXTURE AND AGING THE RESULTING MIXTURE BY ALLOWING SAID MIXTURE TO STAND AT ROOM TEMPERATURE FOR A PERIOD OF AT LEAST 12 HOURS, AND THEN MIXING THE MILLED AND AGED MIXTURE WITH SAID RUBBER (B), AND MILLING THE RESULTANT MIXTURE.
 2. In a process for the preparation of a carbon black-containing rubber composition wherein the rubber components of said composition consist of 80-40 percent by weight of butadiene-acrylonitrile copolymer rubber (A) and 20-60 percent by weight of isoprene-acrylonitrile copolymer rubber (B) or 60-30 percent by weight of butadiene-styrene copolymer rubber (A) and 40-70 percent by weight of natural rubber (B), the improvement wherein a rubber composition having improved green strength is obtained by first compounding said rubber (A) with carbn black, milling the resulting mixture and aging the resulting mixture by allowing said mixture to stand at room temperature for a period of at least 12 hours, and then mixing the resulting gelled carbon black master batch with said rubber (B) and carbon black, and milling the resultant mixture, the total amount of carbon black used being 40-150 parts by weight per 100 parts by weight of the total rubber.
 3. In a process for the preparation of a carbon black-containing rubber composition wherein the rubber components of said composition consist of 80-40 percent by weight of butadiene-acrylonitrile copolymer rubber (A) and 20-60 PERCENT by weight of isoprene-acrylonitrile copolymer rubber (B) or 60-30 percent by weight of butadiene-styrene copolymer rubber (A) and 40-70 percent by weight of natural rubber (B), the improvement wherein a rubber composition having improved green strength is obtained by first mixing a latex of said rubber (A) with a dispersion of carbon black, coagulating, drying and milling the resulting mixture and aging the resulting mixture by allowing said mixture to stand at room temperature for a period of at least 12 hours, and then mixing the resulting gelled carbon black master batch with said rubber (B) and carbon black, and milling the resultant mixture, the total amount of carbon black used being 40-150 parts by weight per 100 parts by weight of the total rubber. 